As the means of controlling exposure in a photographic printer or a copying machine, there has been recently known a system using a liquid crystal element. When gradation is obtained by using the liquid crystal element, transmitted light is modulated by utilizing a gradation characteristic of the liquid crystal element itself. The gradation characteristic of the liquid crystal itself depends upon a threshold characteristic. The more gradually the threshold characteristic varies, much gradations to be obtained.
When a gradation image is exposed, it is preferable that it should be exposed with much gradations as possible. However, only 32 gradations or thereabouts are obtained in the present circumstances, and only 64 gradations or thereabouts are obtained in an experimental stage. On the other hand, in a photographic printer, multi-gradation exposure of 1024 gradations is required.
Further, since a liquid-crystal operation mode in which the liquid crystal itself has a middle tone characteristic is restricted, the degree of design freedom of a liquid crystal element is limited. As a result, it is difficult to provide a desirable modulating element at a low cost.